Swedish Pat. No. 341,956 ("Mechanism for testing protective relays and automatic equipment") and ABB Relay's pamphlet B03-9510E described in detail a relay test system according to the above. It comprises a fixed test switch with room for a plurality of contact units for different test functions. The test switch is suitably mounted in the frame in which the relays of the protective relay system are mounted. Depending on the functions in question, connection is performed from the relays of the protective system to the contacts on the rear side of the test switch for connection to the respective contact unit.
The test system further comprises a movable test-plug handle which is provided with test plugs adapted, after having been inserted into the different contact units, to perform the function of the contact unit in question. When the handle is inserted into the test switch , it is guided by two resilient tongues. Associated with the test-plug handle are also connectible leads for supply and measurement of the relays, the state of which is to be checked.
Otherwise, the test system also comprises certain other parts, such as a block-plug handle, plugs for different functions, etc., which do not require description in order to describe the invention.
As will have been clear from the above, in a protective relay with a test switch the circuits for tripping of circuit-breakers etc. extend via the test switch in order to be disconnected there when the test-plug handle is inserted into the test switch, i.e. when the protective system is to be tested. This enables testing, which is a fault simulation involving a study of the output signals of the protective relay, to be performed without, for example, a circuit-breaker tripping.
As indicated by way of introduction, however, in certain situations problems may arise which could be prevented if an external indication that relay testing is in progress is available.
In this connection, it is highly desirable to obtain this information before the test process is started. At the same time it is also desirable that initiation of this information takes place immediately before the start of the testing. What is possible to achieve in this respect is a situation where the information is generated as the test-plug handle is insertd into the test switch, however with the above reservation, i.e. that the information is obtained before the test process is started. A device accordng to the invention covers both of these desired objects.
There are several reasons for the need of information that testing is in progress. Electric power distribution systems are often designed in such a way that there is a central power supply board and a works management responsible for a plurality of unmanned distribution stations. If protective relays in any of these normally unmanned stations are to be tested as regards, for example, overcurrent, undervoltage, blocking function, etc., in recent coordinated protective relay systems a signal indicating overcurrent, undervoltage, etc. is often transmitted via special communication links between the electronics of the protective relays and from the protective relays to other control equipment. The information in these links is digital and sometimes optical and cannot, therefore, pass via the test switch and be disconnected during testing. The consequence of this is that the information indicates a fault which the protective relay attemps to act upon, when, as a matter of fact, it is a question of testing in progress.